1. Field of the Invention
The invention has to do with the production on a continuous basis of highly pure zirconium and/or hafnium vapors for use in the further production of zirconium or hafnium metal.
2. Description of the Prior Art
In the production of zirconium or hafnium metal, as used structurally in the nuclear industry, from a source material such as zircon sand, it has been common practice to employ a purification process involving the controlled sublimation of a solid tetrachloride of zirconium or hafnium or both. In that process, trace impurities such as carbon and other chlorides, e.g. those of iron, uranium, phosphorous, and aluminum, are largely eliminated by separating tetrachlorides of zirconium and/or hafnium from oxides and oxychlorides of zirconium and/or hafnium. This is normally accomplished in a fluidized bed or flash type of subliming vessel, which is externally heated and into which the solid tetrachloride as a powder is continuously charged by means of an auger or pneumatic transporting device. As the tetrachloride powder reaches its subliming temperature, the vapor therefrom leaves the vessel through a filter located at the top of the vessel. Impurities such as those mentioned above, having a lower vapor pressure at the operating temperature, are partially left behind in the subliming vessel.
This process suffers from the fact that the equipment employed is difficult to operate. It is difficult to feed the tetrachloride powder into the vessel, because the tetrachloride vapor condenses on the feeder and plugs the operation and the subliming vessel has poor heat transfer characteristics requiring large surface areas to process small quantities of the tetrachloride; also, the impurity separation is not complete, because impurities vaporize at the operating temperature.
A fused salt system for producing a zirconium/hafnium vapor by sublimation of a zirconium/hafnium tetrachloride powder is described in U.S. Pat. No. 3,098,722 of July 23, 1963, which employs a melting vessel and a separate vaporizing vessel. U.S. Pat. No. 4,021,531 of May 3, 1977, discloses that combined zirconium/hafnium tetrachloride vapors from a sublimation unit may be fed to an extractive distillation procedure for the separation of hafnium from the zirconium. A fused salt system does have advantages in that it produces a tetrachloride vapor of greater purity than is obtained by the commonly used process described above and the vapor is free of nitrogen and oxygen, which is highly desirable. This is a result of the complexing of the impurities with the fused salt, making them much less volatile.